While volatile memories, such as dynamic random access memories (DRAMs) provide a method to store information, many applications today make use of non-volatile memories devices that will retain information even when power is removed.
Presently, virtually all nonvolatile memories are some type of read only memories (ROMs). The first group of nonvolatile memories consists of ROMs in which data is entered during manufacturing that cannot be subsequently altered. These devices are known as masked ROMS. The next group consists of memories whose data can be entered by the user. This group is known as a programmable ROM or PROM in which data can only be entered once.
In the remaining ROM types, data can be erased as well as entered. In one class of erasable ROMs, the cells must be exposed to a strong ultraviolet light in order for stored data to be erased. These ROMs are called erasable programmable ROMs, or EPROMs. In the final type, data can be electrically erased as well as entered in the device and are referred to as EEPROMs and flash EEPROMs.
There is one common element among the programmable structures that reside in PROMS, EPROMs, EEPROMs, Flash EEPROMs, and a RAM having programmable options available. That commonality is the need for an external programming scheme to either program in data or to program a one-time programmable element that is used to select certain options or to allow circuit repair of semiconductor device such as in DRAMs.
Conventionally, to program an element (in PROMs, DRAMs) or to charge a floating gate (in the EPROM family) a large external voltage typically in the neighborhood of 12-30 V is applied across the appropriate nodes to either rupture a fuse or antifuse or to charge a floating gate, whatever the case may be.
A new method of programming an antifuse element by using transistor snap-back is disclosed in copending application #821,501, by the same inventor, as herein incorporated by reference (Snap-back is a form of field effect transistor breakdown, a condition well known to one skilled in the art, where a positive-feedback mechanism exist between the substrate-source pn junction, thus allowing a large surge of current to flow through the source/drain terminals). A transistor operating in a snap-back condition theortically will allow an infinite amount of current to flow through the transistor's channel region and is limited only by the power source capabilities. And very importantly, studies have shown that a transistor operating during snap-back is not damaged if operated under AC pulse conditions. Using one of the programming options of the present invention, is an optimal way to take advantage of transistor snap-back for programming such elements as an antifuse as described in copending application #821,501, whereby the resistivity of a programmed antifuse element will be reduced to the desirable level of several hundred ohms.
The present invention introduces a method to program the above mentioned programmable integrated circuits by varying the programming pulse width and/or by ramping the pulse amplitude up or down to achieve optimum programming effect from the programming pulse.